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The Earth was a very different place 4 billion years ago: The
planet was much hotter — uninhabitable for even the hardiest
forms of life — and the familiar landscapes we know today were
completely absent.

During this time, the so-called Archean Eon, the
first continents were beginning to coagulate at the Earth's
surface. How they got there has been one of the longest standing
and most debated questions for geoscientists.

Now a team from Germany thinks it may have an answer: Rather than
boiling up from the mantle, the earliest continents oozed from
crust near the
Earth's surface.

"This might sound a little unspectacular, but it may have serious
implications as to how we think about the face of the early
Earth," said team member Thorsten Nagel, a geologist at the
University of Bonn.

Modeling molten mixtures

To study the oldest continental rocks, Nagel's team first had to
find some.

They focused on southwestern Greenland's Isua region because it's
home to some of the planet's oldest and most studied ancient
rocks. What's more, Isua's old continental rocks are found next
to old basalts, types of rock that makes up the ocean floor.
[ World's Most Famous
Rocks ]

Finding the two types of old rock together gave Nagel's team a
chance to compare their makeup and figure out how the basalts
could have melted to form the continental rocks. Basalts (and all
other rocks) form different "melts" — or molten mixtures — at
different temperatures and pressures, so the final composition of
a rock is a clue to how deep within the Earth it formed.

Nagel and his team ran sets of computer experiments to see what
would happen to the old Isua basalts if they melted at different
depths. They modeled basalt melts at 62 miles (100 kilometers)
deep — where most geoscientists think the oldest continental
rocks formed — and melts at 19 to 25 miles (30 to 40 km) deep.

The answers they got were surprising.

"A very simple model suddenly explained all the geochemical
data," said Carsten Münker, a geologist from the University of
Cologne, who co-authored the study.

To subduct, or to ooze?

Using the deeper melt model — the one that most geologists
currently favor — the predicted makeup of the old continental
rocks did not match what's found at Isua. But when the team
modeled melting basalts at the shallower depths, the compositions
matched perfectly.

"The results could not be better," Nagel told OurAmazingPlanet.
"One experiment resulted in a scarily good reproduction" of the
old Isua continental rocks.

The real difference between the two models is that, in the deeper
one, the early continents have to form within the mantle at a
subduction zone, where one tectonic plate plunges into the mantle
under another. But in the shallower model, the early continents
"ooze" out at the surface of the Earth, completely within the
crust, not the mantle.

The new shallower model opens the door to a fundamental question:
Did the early Earth even have subduction zones?

Nagel isn't sure whether it did, but the answer to that question
could change a lot of what scientists think they know about the
early Earth.

"Our present-day planet and its topography, climate and the
distribution of land and sea is shaped by
modern plate tectonics," Nagel said. "The early Earth was
certainly hotter than today, and this might have had fundamental
consequences on how plate tectonics worked, in a
hard-to-predict way."

"The way the early Earth worked might still hold a lot of
surprises for us," he added.